Use inline {@return ...} tag for Javadoc summaries missing from the public API

A number of public/protected members in `core/` have Javadoc comments that are missing a summary but contain an `@return` tag. Javadoc provides the inline `{@return ...}` tag that can be used to satisfy both.

For JavaDocs missing a summary, but providing an `@return` tag, I want to replace all `@return ...` tags with the inline tag and move the inline tag to the summary position.

I've already made the changes and plan to open a PR.

Inspect Javadoc comments for public and protected members under core/ that lack a summary but contain an @return tag. Confirm that each affected tag is converted to the inline form and placed in the summary position; completion means all matching comments are updated, though the issue notes that changes are already made and a PR is planned.

Written by the indexing model from the issue text.